Key management practices controlling yield performance and nitrate leaching in a grass-clover based organic crop rotation

Organic farming is frequently discussed as a strategy for mitigating nitrate leaching, especially in water protection areas. The effectiveness of nitrogen (N) management practices within organic crop rotations requires evaluation to balance yield performance with nitrate leaching. A field trial was conducted from 2020 to 2024 in North-West Germany, to assess the impact of grass-clover management on yield performance and nitrate leaching in an organic rotation of grass-clover-(cover crop (CC))-silage maize–winter barley. Grass-clover was mulched as green manure without N fertilization in subsequent crops or grass-clover was harvested. The N removed by harvesting was returned with biogas digestate fertilization (BDF) to subsequent crops. Additionally, two termination times as ploughing grass-clover in fall with establishment of a winter CC or ploughing in spring were implemented. Nitrate concentrations in leachates were analyzed biweekly during winter. Nitrate leaching was 65% higher after fall-terminated grass-clover compared with overwintered stands. Spring-terminated grass-clover enhanced maize yields irrespective of BDF. The combination of BDF to maize and spring-terminated grass-clover increased nitrate leaching after maize compared to both spring termination without BDF and fall termination with BDF. For winter barley, nitrate leaching was unaffected by grass-clover termination or BDF, but grain yields increased significantly with BDF. Grass-clover termination timing and BDF allocation across subsequent crops were found to be key levers for optimizing N management. The findings suggest that BDF can be reduced for the first crop after spring-terminated grass-clover and redirected to the second crop, thereby improving yield performance while reducing the risk of nitrate leaching.